Dust shield for plunger operated electrical switch

ABSTRACT

A plunger-operated electrical switch is provided with an annular retaining groove surrounding the plunger opening and a one-piece flexible dust shield of inverted-cup-shape disposed over the plunger and having a thin membrane cover portion integral with a radially inward extending annular lip that terminates in an annular retaining bead sealably wedged in the retaining groove. The membrane cover portion of the dust shield cooperates with the annular lip in the manner of a bellows to allow unrestricted plunger travel until it is nearly flush with the switch housing and with only minimal increase in switch height and with negligible increase in operating force.

United States Patent [191 Halbach et al.

[4 June 28, 1974 DUST SHIELD FOR PLUNGER OPERATED ELECTRICAL SWITCH [75] Inventors: Edward A. Halbach, Milwaukee;

Paul M Gallatin, Brookfield, both of Wis.

[73] Assignee: Allis-Chalmers Corporation,

Milwaukee, Wis.

[22 Filed: Aug. 27, 1973 [21] Appl. No.: 392,124

[g2] rigg ..1 .z 09 ;02 51] Int. Cl. ..H0lh 9/04 [58] Field of Search 200/168 G, 168 C, 168 D; 277/212 C, 212 FB 56 References Cited UNITED STATES PATENTS 2,451,176 10/1948 Schellman 200/168 G 2,584,883 2/1952 Karsch 200/168 G 2,854,536 9/1958 Beer ZOO/168 G 3,046,591 7/1962 Prohaska et a1. 200/168 G 3,296,404 1/1967 Stevens 200/168 G 3,316,379 Clark et al. 200/168 G FOREIGN PATENTS OR APPLICATIONS 124,178 8/1931 Gennany 200/168 G 334,436 1/1959 Switzerland 200/168 G Primary Examiner-B. Dobeck Assistant EraminerWilliam J. Smith Attorney, Agent, or Firm-Lee H. Kaiser [5 ABSTRACT A plunger-operated electrical switch is provided with an annular retaining groove surrounding the plunger opening and a one-piece flexible dust shield of inverted-cup-shape disposed over the plunger and having a thin membrane cover portion integral with a radially inward extending annular lip that terminates in an annular retaining bead sealably wedged in the retaining groove. The membrane cover portion of the dust shield cooperates with the annular lip in the manner of a bellows to allow unrestricted plunger travel until it is nearly flush with the switch housing and with only minimal increase in switch height and with negligible increase in operating force.

6 Claims, 3 Drawing Figures DUST SHIELD FOR PLUNGER OPERATED ELECTRICAL SWITCH BACKGROUND OF THE INVENTION This invention relates to electrical circuit makers and breakers and in particular to a plunger-operated electrical switch having an improved dust shield to prevent entry of environmental contaminants into the contact cavity.

If air contaminants such as dust and vapors enter the housing of a miniature electrical switch such as a snapacting switch, contacts may rapidly become fouled, thereby causing electrical failure of the switch. The problem of sealing the opening through the switch housing for the reciprocable switch plunger is particularly difficult. In an attempt to seal the contacts from air contaminants, electrical switches have been mounted in auxiliary housings which are gasketed or hermetically sealed, and auxiliary means are provided to operate the switch contacts through the housings, but such prior art structures require increased operating force and occupy greater space and appreciably increase the cost of the switch.

SUMMARY OF THE INVENTION According to the invention, a plunger-operated electrical switch is provided with a flexible, one-piece, inverted cup-shaped dust shield positioned over the plunger which seals the contacts from air contaminants entering the housing through the opening for the plunger. The dust shield has a membrane cover portion of greater diameter than the retaining groove and an integral lip projecting radially inward and downwardly from the margin of the cover in a bellows-like manner which permits appreciable deflection of the membrane cover so that the switch plunger can have substantial travel with negligible increase in operating force and with minimal increase in switch height. An annular retaining groove is provided in the exterior surface of the switch housing surrounding the plunger opening, and the inwardly projecting lip of the dust shield terminates in an annular retaining bead which is sealably wedged with a compression fit into the retaining groove. The retaining bead is preferably of generally triangular cross section with a flat top surface that facilitates pushing the retaining bead into the retaining groove. Any pull on the membrane cover portion to remove the dust shield from the housing is transmitted as a tensile force through the lip to the retaining bead and tends to pivot the bead cross section in a direction which augments the compressive forces that hold the retaining bead within the retaining groove.

It is the primary object of the invention to provide an improved dust shield construction for a plungeroperated electrical switch which effectively seals the switch contacts from air contaminants without measurably increasing the operating force for the plunger or the space occupied by the switch.

It is a further object of the invention to provide an improved one-piece dust shield construction adapted for use with a commercially available plunger-operated electrical switch which allows unrestricted full plunger travel until it is nearly flush with the switch housing.

A still further object of the invention is to provide such an improved dust shield having a novel crosssectional configuration which facilitates installation of the shield on the switch and which augments the retentive forces holding the dust shield on the switch housing when a pull is exerted on the dust shield to remove it from the housing.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the invention will be more readily apparent from the following detailed description when considered in conjunction with the accompanying drawing wherein:

FIG. 1 is a plan view of a preferred embodiment of the invention wherein the improved dust shield seals the contacts of a miniature switch from air contaminants;

FIG. 2 is a front view, partly in cross section, of the FIG. 1 embodiment representing the switch contacts schematically in dotted lines; and

FIG. 3 is a view, partly in cross section, of the dust shield.

DETAILED DESCRIPTION Referring to the drawing, a plunger-operated miniature electrical switch, which may be a commercially available switch of conventional construction, has a switch housing 10 of insulating material enclosing a switch mechanism 11 which is actuated by a plunger 12. The switch mechanism 11 does not constitute part of the present invention and is only represented schematically in the drawing and will not be described for sake of simplicity. Plunger 12 is reciprocable axially through an opening 14 in housing 10 to actuate switch mechanism 11 between a released position, wherein plunger 12 protrudes above the exterior surface 15 of housing 10, and an operated position wherein plunger 12 is substantially flush with the exterior surface 15 of housing 10.

The dust shield of the invention prevents air contaminants from entering through plunger opening 14 into the contact cavity 16 within housing 10 where such contaminants could foul the electrical contacts of switch mechanism 11. An annular retaining groove 17 is fabricated by machining or molding in the exterior surface 15 of housing 10 concentric with plunger opening 14, and preferably the walls defining retaining groove 17 are parallel to the axis of reciprocation of plunger 12 so that the cost of fabrication is minimum.

Dust shield 20 is of generally inverted U-shaped configuration and is disposed over plunger 12 and held on switch housing 10 by an integral retaining bead 27 compressed within retaining groove 17 to seal the contact cavity 16 from air contaminants. Dust shield 20 is preferably molded as a single piece and is of highly flexible elastomeric material such as silicone rubber. Dust shield 20 has a membrane cover portion 23 of greater diameter than retaining groove 17, an annular lip 25 integral with cover portion 23 and extending radially inwardlyand downwardly from the outer margin of cover portion 23 in a manner analogous to a bellows to permit substantial travel of plunger 12 with minimal increase of switch operating force and switch height. Lip 25 terminates in an integral annular retaining bead 27 of generally triangular cross section compressed within retaining groove 17.

Retaining bead 27 has a flat upper annular surface 28 to facilitate pressing bead 27 into retaining groove 17. Any downward push exerted on flexible membrane cover portion 23 is transmitted to flat annular surface 28 to urge retaining bead 27 into retaining groove 17. In the preferred embodiment retaining bead 27 is generally triangular in cross section with the radially outer surface 30 of bead 27 (where lip 25 joins bead 27) extending parallel to the direction of reciprocation of plunger 12 and constituting one of the legs of the cross section triangle. The radially inner surface of retaining bead 27 has a radially inward projecting circumferential boss 32 which blends into an upwardly inclined circumferential surface 34 that constitutes a second leg of the bead cross section triangle. Retaining bead 27 is wider in a radial direction at boss 32 than the radial width of retaining groove 17. Consequently, any upward force exerted on dust shield 20 to remove it from switch housing is transmitted through lip 25 as a tensile force which tends to pivot the triangular-in-crosssection bead 27 about boss 32. Such upwardly directed tensile force tends to urge the material in the radially outer position of the bead cross section with greater compressive force against the radially outer surface of retaining groove 17, thereby increasing the retentive force holding dust shield on switch housing 24.

An alternative embodiment wherein the circumferential boss (such as 32 in the preferred embodiment) is on the radially outer face of the retaining bead was also found to have excellent retentive force.

It is apparent that an undercut retaining groove which more closely conforms to the cross section of the retaining bead 27 would increase the retention force, but fabrication of such an undercut groove adds sub stantially to cost.

While only a single embodiment of the invention has been illustrated and described, it should be understood that we do not intend to be limited to the single embodiment, for many modifications and variations thereof will be obvious to those skilled in the art.

The embodiment of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In combination with a plunger operated electrical switch having an insulated housing, a switching mechanism within said housing, and a plunger extending through a cooperating opening in said housing for actuating said switching mechanism and being reciprocable I through said opening between a released switch mechanism position, wherein said plunger protrudes above said housing, and an operated switch mechanism position, wherein said plunger is substantially flush with the exterior surface of said housing, the exterior surface of said housing having an annular retaining groove in surrounding relation to said opening, a one-piece dust shield of flexible insulating material and of generally inverted cup-shaped configuration disposed over said plunger and having a thin membrane cover portion of greater diameter than said groove integral with an annular lip extending radially inward from the outer margin of said membrane cover portion and terminating in an annular retaining bead of greater radial thickness than the radial width of said retaining groove sealably wedged in said retaining groove to affix said lip to said housing so that said membrane cover portion cooperates with said radially inward extending lip in the manner of a bellows to permit substantial travel of said plunger between said released switch mechanism position and said operated switch mechanism position and with only minimal increase in switch height, the surface retaining bead is generally triangular in cross sectionv with one radially facing surface of said retaining bead generally parallel to the direction of reciprocation of said plunger and constituting one leg of the cross section triangle.

4. In the combination of claim 1 wherein the radially facing surfaces of said retaining groove are generally parallel to the direction of reciprocation of said plunger, one radially facing surface of said annular retaining bead is generally parallel to the direction of reciprocation of said plunger and the other radially facing surface has a circumferential, radially extending boss thereon, the radial dimension of said retaining bead at said boss being greater than the radial width of said retaining groove, whereby a pull on said dust cover to remove it from said switch housing augments the compressive forces tending to hold said retaining bead within said retaining groove.

5. An electrical switch assembly comprising, in combination, a housing, a switching mechanism within said housing, a switch mechanism operating plunger extending through an opening in said housing and being reciprocal between a released switch mechanism position, wherein it protrudes above the exterior of said housing, and an operated switch mechanism position, wherein it is approximately flush with said exterior surface of said housing, an annular retaining groove in said exterior surface of said housing concentric with said plunger opening, a one-piece dust shield of elastomeric material having a thin membrane cover portion disposed above and being of greater diameter than said groove integral with an annular lip extending radially inward from the outer margin of said cover portion so that said cover portion and said lip cooperate in the manner of a bellows to permit substantial travel of said plunger between said released and operated positions, said lip terminating at its radially inner end in an integral annular retaining bead and having a flat annular upper surface to facilitate pushing said bead into said retaining groove and also having a generally triangular cross section with one radially facing surface of the bead cross section extending parallel to the direction of reciprocation of said plunger and constituting one of the legs of the cross section triangle.

6. In the combination of claim 5 wherein the walls defining said annular groove are generally parallel to the direction of reciprocation of said plunger, said one radially facing surface of said bead cross section is the radially outer surface thereof and said lip joins said bead adjacent said radially outer surface, the radially inner surface of said retaining bead having a circumferential boss thereon, whereby a pull on said membrane portion to remove said shield from said switch housing is transmitted through said lip and tends to pivot said bead about said boss and increase the compressive force holding said retaining bead within said retaining 

1. In combination with a plunger operated electrical switch having an insulated housing, a switching mechanism within said housing, and a plunger extending through a cooperating opening in said housing for actuating said switching mechanism and being reciprocable through said opening between a released switch mechanism position, wherein said plunger protrudes above said housing, and an operated switch mechanism position, wherein said plunger is substantially flush with the exterior surface of said housing, the exterior surface of said housing having an annular retaining groove in surrounding relation to said opening, a onepiece dust shield of flexible insulating material and of generally inverted cup-shaped configuration disposed over said plunger and having a thin membrane cover portion of greater diameter than said groove integral with an annular lip extending radially inward from the outer margin of said membrane cover portion and terminating in an annular retaining bead of greater radial thickness than the radial width of said retaining groove sealably wedged in said retaining groove to affix said lip to said housing so that said membrane cover portion cooperates with said radially inward extending lip in the manner of a bellows to permit substantial travel of said plunger between said released switch mechanism position and said operated switch mechanism position and with only minimal increase in switch height, the surface of said annular retaining bead facing said cover portion being flat and generally parallel to the exterior surface of said switch housing to facilitate pushing said retaining bead into said retaining groove.
 2. In combination of claim 1 wherein said dust cover is of a stable resilient elastomeric material having high resistance to tearing.
 3. In the combination of claim 1 wherein said annular retaining bead is generally triangular in cross section with one radially facing surface of said retaining bead generally parallel to the direction of reciprocation of said plunger and constituting one leg of the cross section triangle.
 4. In the combination of claim 1 wherein the radially facing surfaces of said retaining groove are generally parallel to the direction of reciprocation of said plunger, one radially facing surface of said annular retaining bead is generally parallel to the direction of reciprocation of said plunger and the other radially facing surface has a circumferential, radially extending boss thereon, the radial dimension of said retaining bead at said boss being greater than the radial width of said retaining groove, whereby a pull on said dust cover to remove it from said switch housing augments the compressive forces tending to hold said retaining bead within said retaining groove.
 5. An electrical switch assembly comprising, in combination, a housing, a switching mechanism within said housing, a switch mechanism operating plunger extending through an opening in said housing and being reciprocal between a released switch mechanism position, wherein it protrudes above the exterior of said housing, and an operated switch mechanism position, wherein it is approximately flush with said exterior surface of said housing, an annular retaining groove in said exterior surface of said housing concentric with said plunger opening, a one-piece dust shield of elastomeric material having a thin membrane cover portion disposed above and being of greater diameter than said groove integral with an annular lip extending radially inward from the outer margin of said cover portion so that said cover portion and said lip cooperate in the manner of a bellows to permit substantial travel of said plunger between said released and operated positions, said lip terminating at its radially inner end in an integral annular retaining bead and having a flat annular upper surface to facilitate pushing said bead into said retaining groove and also having a generally triangular cross section with one radially facing surface of the bead cross section extending parallel to the direction of reciprocation of said plunger and constituting one of the legs of the cross section triangle.
 6. In the combination of claim 5 wherein the walls defining said annular groove are generally parallel to the direction of reciprocation of said plunger, said one radially facing surface of said bead cross section is the radially outer surface thereof and said lip joins said bead adjacent said radially outer surface, the radially inner surface of said retaining bead having a circumferential boss thereon, whereby a pull on said membrane portion to remove said shield from said switch housing is transmitted through said lip and tends to pivot said bead about said boss and increase the compressive force holding said retaining bead within said retaining groove. 